Junk food appears to be almost as addictive as heroin, according to a study conducted by researchers from the Scripps Research Institute and presented at the annual meeting of the Society for Neuroscience.

“This is the most complete evidence to date that suggests obesity and drug addiction have common neuro-biological foundations,” researcher Paul Johnson said.

The researchers fed rats one of three diets: a nutritious diet, a healthy diet with restricted access to junk food, or a diet of unlimited junk food. Junk foods included cheesecake, chocolate, sponge cake and fatty meat. Mice in the third group quickly became obese, while the weight of mice in the first two groups did not change.

To test the effects of junk food on the brain’s pleasure centers — the areas affected by drugs — the researchers electrically stimulated those areas whenever the mice ran on a wheel. The longer a rat ran on the wheel, the more pleasure it would receive.

While rats in the first two groups did not change their wheel-running behavior, the rats eating junk food soon began running on the wheel for longer periods — suggesting that their brain’s pleasure centers had become less sensitive. Consistent with this finding, the rats began eating more and more food, suggesting that their bodies had become desensitized to the pleasure the food was producing.

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The researchers then began exposing rats to painful electric shocks whenever they ate junk food. The rats on the restricted junk food diet quickly stopped eating the food, but the binge-eating rats were undeterred.